This invention generally relates to the construction of fuel cell stacks and, more particularly, to the construction of fuel cells, such as oxygen-ion conducting solid oxide fuel cells and proton conducting ceramic or polymer membrane fuel cells, in which the electrolyte is a solid.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant to generate a direct current. A fuel cell typically includes a cathode material, an electrolyte material, and an anode material. The electrolyte is a non-porous material sandwiched between the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. The electrical interconnect also provides for passageways which allow oxidant fluid to flow past the cathode and fuel fluid to flow past the anode, while keeping these fluids separated. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are typically gases and are continuously passed through separate passageways. Electrochemical conversion occurs at or near the three-phase boundaries of each electrode (cathode and anode) and the electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Fuel cells with solid electrolytes are the most promising technologies for power generation. Solid electrolytes are either ion conducting ceramic or polymer membranes. In the former instance, the electrolyte is typically made of a ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. With such an electrolyte, the anode is oftentimes made of nickel/YSZ cermet and the cathode is oftentimes made of doped lanthanum manganite.
Perhaps the most advanced construction with ceramic membranes is the tubular solid oxide fuel cell based on zirconia. The tubular construction can be assembled into relatively large units without seals and this is its biggest engineering advantage. However, tubular solid oxide fuel cells are fabricated by electrochemical vapor deposition processes, which are slow and costly. The tubular geometry of these fuel cells also limits the specific power density, both on weight and volume bases, to low values. The electron conduction paths are also long and lead to high energy losses due to internal resistance heating. For these reasons, other constructions are actively being pursued.
The common alternative construction to the tubular construction is a planar construction that resembles a cross-flow heat exchanger in a cubic configuration. The planar cross flow fuel cell is built from alternating flat single cell membranes (which are trilayer anode/electrolyte/cathode structures) and bipolar plates (which conduct current from cell to cell and provide channels for gas flow into a cubic structure or stack). The bipolar plates are oftentimes made of suitable metallic materials. The cross-flow stack is manifolded externally on four faces for fuel and oxidant gas management.
The cross-flow or cubic design, however, requires extensive sealing, both in terms of the number of seal interfaces and the linear size of such interfaces. The latter increases with the stack footprint and leads to serious problems if the metal and ceramic cell parts do not have closely matched thermal expansion coefficients. A significant mismatch in the thermal expansion coefficients leads to thermal stresses that can cause catastrophic failure on cool down from the stack operating temperature.
Additional disadvantages of the cross-flow design include the skewed density distribution and, thereby, skewed temperature distribution that is imposed by the flow field. Also, the cross-flow design has stress concentrations at the corners that can have damaging effects. From a cost-effective manufacturing view, manifolding the cubic device in a co-flow configuration is virtually impossible due to the extremely short height of the gas channels.
An alternative to the cross-flow or cubic design has been a radial or co-flow design. For example, U.S. Pat. No. 4,770,955 discloses an annular shaped anode, cathode, and electrolyte sandwiched therebetween. Annular shaped separator plates sandwich the combination of anode, cathode, and electrolyte. The above components each describe two holes and, consequently, two tubes. One tube provides a fuel flow while the other tube provides an oxidant flow. The cathode is protected from direct fuel contact in one tube by a tubular gasket that forms a seal with one separator and electrolyte. The anode is protected from direct oxidant contact in the other tube by another tubular gasket that forms a seal with the other separator and electrolyte. Yet, the design appears to inherently lack good flow control because it is based on the porosity of the electrode and radial length of the porous electrode from the tube. Also, the non-symmetrical position of the tubes results in differing flow path lengths as a function of the central angle with the tube as the center and, thus, non-uniform flow distribution. U.S. Pat. No. 5,589,285 is similar to the foregoing.
In another example of a radial fuel stack design, U.S. Pat. No. 4,490,445 provides alternating circular cells and conductor plates. The cells and plates are provided with holes along their peripheries to create fuel and oxidant inlets and outlets. The conductor plates are provided with circumferential ridges along the edges to provide seals with the cells. The conductor plates also have grooves on opposing faces that provide flow both radially and circumferentially, although primarily the latter. With the potential flow paths being somewhat random, non-uniform flow distribution can be expected.
U.S. Pat. No. 4,910,100 discloses various embodiments of a radial fuel cell stack design that include fuel and oxidant channels in the central area of the stack. Gas holes in the fuel and oxidant channels supply flows across the opposing faces of annular shaped separator plates that are alternately disposed with annular shaped single cells. Guide vanes on the opposing faces of the separator plates direct fuel and oxidant flow from a central area of the stack and towards the peripheral area. It is claimed that the plates and cells may be stacked without gas seals. However, some of the disadvantages to this design include the non-uniform distribution of oxidant and fuel gases due to the use of two internal manifold tubes and the heavy, all-ceramic construction of individual parts.
More recently, in U.S. Pat. No. 5,549,983, a co-flow planar fuel cell stack for solid electrolytes includes an internal manifold having fuel and oxidant cavities. Tubular porous elements surround the manifold for controlling radial fuel and oxidant flows. The tubular porous elements may also be called flow distributor elements or simply flow distributors. Annular, planar cells of anode/electrolyte/cathode are disposed about the porous elements. An annular separator plate is sandwiched between each single cell and each current conductor element. The single cells and separator plates extend at their inner diameters to the inner manifold. Accordingly, a sealant is required to seal the separator plates and single cells to the manifold and porous elements. Notwithstanding its advantages, the design includes a significant number of interfaces to be sealed. Having the individual cells sealed to the porous elements provides the potential for cracking due to sources such as bending stresses and pressure differentials.
Other examples of radial-type fuel stack designs are found in U.S. Pat. Nos. 5,851,689; 5,527,634; and 5,399,442.
As can be seen, there is a need for an improved fuel stack design for solid electrolytes. In particular, there is a need for a radial flow fuel cell stack that minimizes the required sealing in terms of the number of interfaces to be sealed and the length of the interfaces. Also needed is a fuel cell stack that provides a more uniform flow field and, thereby, more uniform current density and temperature distributions. Another need includes ease of manifolding for co-flow and stack arrays in fuel cells having solid electrolytes. A fuel cell stack design is needed that also eliminates 90.degree. corners where damaging stress concentrations can arise.